DIRECTIONAL EFFECTS ON CONVERGENT FLOW TRACER TESTS

Convergent flow tracer tests constitute a convenient way of characteri zing hydraulic parameters in an aquifer. Interpretation of tracer brea kthrough curves from convergent flow tests normally is made under the assumption of radial symmetry. Nevertheless, these curves may display directional dependence; that is when tracers are injected at several p oints located at the same distance, both arrival times and estimated d ispersivities may be significantly different. This result is why some authors attribute a tensorial nature to porosity or, equivalently, tal k about directional porosity when trying to explain the variations in computed porosity depending on the relative orientation of pumping and injection wells. Our main ponit is that this directional effect is no thing but an artifact of an inappropriate selection of a conceptual mo del, where anisotropy (local of statistical) in hydraulic conductivity is not properly characterized. To illustrate this point, we first con sider the situation of a simple homogeneous and anisotropic model of t he medium. We prove analytically that this model leads to arrival time being proportional to the square root of directional hydraulic conduc tivity. Using a stochastic approach, we determine the same directional behavior of arrival time for a locally isotropic hydraulic conductivi ty field with statistical anisotropy caused by an anisotropic correlat ion structure. A statistical anisotropic covariance model for hydrauli c conductivity is consistent with field evidence.